In the case of gas-discharge lamps, it is frequently desired, and in the case of modern gas-discharge lamps mostly mandatory, for the power converted by the gas-discharge lamp to be controlled. Should this not take place, the gas-discharge lamp can be overloaded, as a result of which its service life is shortened or, in the worst case, an immediate defect occurs.
EP 0605052 A1 discloses an electronic ballast for fluorescent lamps which contains a monitoring circuit for the lamp current. In order to generate the lamp AC voltage, use is made of an invertor bridge which is controlled by a voltage-controlled oscillator (VCO). A series resonant circuit is connected to the output of the invertor bridge in order to feed the fluorescent lamp. Via a shunt, a current signal which is fed as voltage to a filter circuit is tapped at the frame-side base point of the gas-discharge lamp. The DC voltage present at the filter output characterizes the current flowing through the fluorescent lamp. The VCO is controlled by said voltage. The VCO and the series resonant circuit connected to the invertor bridge are dimensioned such that the VCO can operate both at the resonant frequency of the series resonant circuit and at higher frequencies. With increasing lamp current and, in particular, when the lamp current becomes too high, the VCO increases its frequency and thereby leads the series resonant circuit out of resonant operation.
The filter circuit required for filtering the current signal is tuned to the frequency response of the fluorescent lamp, in particular with regard to its zeroing. This can lead to problems when various types of lamps are to be used. Also, the filter to be implemented is enough in itself to require a relatively large number of components. Moreover, a certain reaction time can become noticeable in the case of discontinuous transition operations.